Multi-gate devices and method of fabricating the same

ABSTRACT

Provided is a semiconductor device including a semiconductor substrate, a plurality of semiconductor nanosheets, a plurality of source/drain (S/D) features and a gate stack. The semiconductor substrate includes a first fin and a second fin. The first fin has a first width less than a second width of the second fin, and a top surface of the first fin is lower than a top surface of the second fin. The plurality of semiconductor nanosheets are disposed on the first fin and the second fin. The plurality of source/drain (S/D) features are located on the first fin and the second fin and abutting the plurality of semiconductor nanosheets. The gate stack wraps each of the plurality of semiconductor nanosheets.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of and claims thepriority benefit of a prior application Ser. No. 16/805,832, filed onMar. 2, 2020. The prior application Ser. No. 16/805,832 is anapplication of and claims the priority benefit of U.S. Provisionalapplication Ser. No. 62/906,118, filed on Sep. 26, 2019. The entirety ofthe above-mentioned patent application is hereby incorporated byreference herein and made a part of this specification.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experiencedexponential growth. Technological advances in IC materials and designhave produced generations of ICs where each generation has smaller andmore complex circuits than the previous generation. In the course of ICevolution, functional density (i.e., the number of interconnecteddevices per chip area) has generally increased while geometry size(i.e., the smallest component (or line) that can be created using afabrication process) has decreased. This scaling down process generallyprovides benefits by increasing production efficiency and loweringassociated costs. Such scaling down has also increased the complexity ofprocessing and manufacturing ICs.

Recently, multi-gate devices have been introduced in an effort toimprove gate control by increasing gate-channel coupling, reduceOFF-state current, and reduce short-channel effects (SCEs). One suchmulti-gate device that has been introduced is the gate-all-aroundtransistor (GAA). The GAA device gets its name from the gate structurewhich can extend around the channel region providing access to thechannel on two or four sides. The GAA devices are compatible withconventional complementary metal-oxide-semiconductor (CMOS) processesand their structure allows them to be aggressively scaled whilemaintaining gate control and mitigating SCEs. In conventional processes,the GAA devices provide a channel in a stacked nanosheet configuration.Integration of fabricating the GAA features around stacked nanosheetscan be challenging.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIGS. 1A and 1B illustrate a flow chart of a method 100 for formingmulti-gate devices according to various aspects of the presentdisclosure.

FIGS. 2 to 5 are fragmentary cross-sectional views of an examplesemiconductor device at various stages of fabrication in accordance withsome embodiments of the present disclosure.

FIGS. 6A to 15A illustrates a three-dimensional perspective view of anexample semiconductor device at other various stages of fabrication inaccordance with some embodiments of the present disclosure.

FIGS. 6B to 15B are fragmentary cross-sectional views along the I-I′line of the semiconductor device in FIGS. 6A to 15A at other variousstages of fabrication according to some embodiments of the presentdisclosure.

FIGS. 6C to 15C are fragmentary cross-sectional views along the II-II′line of the semiconductor devices in FIGS. 6A to 15A at other variousstages of fabrication according to some embodiments of the presentdisclosure.

FIGS. 11D and 12D/12E are fragmentary cross-sectional views along theII-II′ line of the semiconductor devices in FIGS. 11A and 12A at othervarious stages of fabrication according to alternative embodiments ofthe present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the disclosure.Specific examples of components and arrangements are described below tosimplify the present disclosure. These are, of course, merely examplesand are not intended to be limiting. For example, the formation of afirst feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

In addition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.Moreover, the formation of a feature on, connected to, and/or coupled toanother feature in the present disclosure that follows may includeembodiments in which the features are formed in direct contact, and mayalso include embodiments in which additional features may be formedinterposing the features, such that the features may not be in directcontact. In addition, spatially relative terms, for example, “lower,”“upper,” “horizontal,” “vertical,” “above,” “over,” “below,” “beneath,”“up,” “down,” “top,” “bottom,” etc. as well as derivatives thereof(e.g., “horizontally,” “downwardly,” “upwardly,” etc.) are used for easeof the present disclosure of one features relationship to anotherfeature. The spatially relative terms are intended to cover differentorientations of the device including the features. Still further, when anumber or a range of numbers is described with “about,” “approximate,”and the like, the term is intended to encompass numbers that are withina reasonable range including the number described, such as within +/−10%of the number described or other values as understood by person skilledin the art. For example, the term “about 5 nm” encompasses the dimensionrange from 4.5 nm to 5.5 nm.

The present disclosure is generally related to semiconductor devices andthe fabrication thereof, and more particularly to multi-gatetransistors. Multi-gate transistors include those transistors whose gatestructures are formed on at least two-sides of a channel region. Thesemulti-gate devices may include a p-type metal-oxide-semiconductor deviceor an n-type metal-oxide-semiconductor multi-gate device. Specificexamples may be presented and referred to herein as FINFET, on accountof their fin-like structure. Also presented herein are embodiments of atype of multi-gate transistor referred to as a gate-all-around (GAA)device. A GAA device includes any device that has its gate structure, orportion thereof, formed on 4-sides of a channel region (e.g.,surrounding a portion of a channel region). Devices presented hereinalso include embodiments that have channel regions disposed in nanosheetchannel(s), bar-shaped channel(s), and/or other suitable channelconfiguration. Presented herein are embodiments of devices that may haveone or more channel regions (e.g., nanosheets) associated with a single,contiguous gate structure. However, one of ordinary skill wouldrecognize that the teaching can apply to a single channel (e.g., singlenanosheets) or any number of channels. One of ordinary skill mayrecognize other examples of semiconductor devices that may benefit fromaspects of the present disclosure.

Conventionally in a GAA process flow, epitaxial S/D features and a gatestack are separated by inner spacers. The inner spacers are formed bydielectric material deposition and an etch process to fill in cavitiesformed in epitaxial stack. However, inner spacer residues remain onfine-sized fins during the formation of the inner spacers, therebyblocking surfaces of the fins in source/drain spaces. As a result,epitaxial S/D features cannot grow from the surface of the fins, andtherefore have more merge defects and poor quality. The presentdisclosure is to remove the inner spacer residue so as to provide highquality epitaxial S/D features.

FIGS. 1A and 1B illustrate a flow chart of a method 100 for formingmulti-gate devices according to various aspects of the presentdisclosure. As used herein, the term “multi-gate device” is used todescribe a device (e.g., a semiconductor transistor) that has at leastsome gate material disposed on multiple sides of at least one channel ofthe device. In some examples, the multi-gate device may be referred toas a GAA device having gate material disposed on four sides of at leastone channel member of the device. The channel member may be referred toas “nanosheet” or “semiconductor nanosheet,” which is used herein todesignate any material portion with nanoscale, or even microscaledimensions, and having an elongate shape, regardless of thecross-sectional shape of this portion. Thus, this term designates bothcircular and substantially circular cross-section elongate materialportions, and beam or bar-shaped material portions including for examplea cylindrical in shape or substantially rectangular cross-section.

FIGS. 1A and 1B will be described below in conjunction with FIGS. 2 to15C. FIGS. 2 to 5 are fragmentary cross-sectional views of asemiconductor device 200 at various stages of fabrication according tosome embodiments of method 100. FIGS. 6A to 15A are diagrammaticperspective views of a semiconductor device 200 at other various stagesof fabrication according to some embodiments of the method 100. FIGS. 6Bto 15B are fragmentary cross-sectional views along the I-I′ line of thesemiconductor devices 200 in FIGS. 6A to 15A at other various stages offabrication according to some embodiments of the method 100. FIGS. 6C to15C are fragmentary cross-sectional views along the II-II′ line of thesemiconductor devices 200 in FIGS. 6A to 15A at other various stages offabrication according to some embodiments of the method 100. FIGS. 11Dand 12D/12E are fragmentary cross-sectional views along the II-II′ lineof the semiconductor devices 200 in FIGS. 11A and 12A at other variousstages of fabrication according to alternative embodiments of the method100. The method 100 is merely an example, and is not intended to limitthe present disclosure beyond what is explicitly recited in the claims.Additional steps may be provided before, during, and after method 100,and some of the steps described can be moved, replaced, or eliminatedfor additional embodiments of method 100. Additional features may beadded in the semiconductor device depicted in FIGS. 2 to 15C and some ofthe features described below can be replaced, modified, or eliminated inother embodiments of the semiconductor device.

As with the other method embodiments and exemplary devices discussedherein, it is understood that parts of the semiconductor device 200 maybe fabricated by a CMOS technology process flow, and thus some processesare only briefly described herein. Further, the exemplary semiconductordevices may include various other devices and features, such as othertypes of devices such as additional transistors, bipolar junctiontransistors, resistors, capacitors, inductors, diodes, fuses, staticrandom access memory (SRAM) and/or other logic circuits, etc., but issimplified for a better understanding of the inventive concepts of thepresent disclosure. In some embodiments, the exemplary devices include aplurality of semiconductor devices (e.g., transistors), including PFETs,NFETs, etc., which may be interconnected. Moreover, it is noted that theprocess steps of method 100, including any descriptions given withreference to FIGS. 2-15C, as with the remainder of the method andexemplary figures provided in this disclosure, are merely exemplary andare not intended to be limiting beyond what is specifically recited inthe claims that follow.

At operation 102, the method 100 (FIG. 1A) provides a substrate 202, asshown in FIG. 2 . In some embodiments, the substrate 202 may be asemiconductor substrate such as a silicon (Si) substrate. The substrate202 may include various layers, including conductive or insulatinglayers formed on a semiconductor substrate. The substrate 202 mayinclude various doping configurations depending on design requirementsas is known in the art. For example, different doping profiles (e.g.,p-well, n-well) may be formed on the substrate 202 in regions designedfor different device types (e.g., n-type field effect transistors(NFET), p-type field effect transistors (PFET)). The suitable doping mayinclude ion implantation of dopants and/or diffusion processes, such asboron (B) for the p-well and phosphorous (P) for the n-well. In someembodiments, the substrate 202 includes a single crystallinesemiconductor layer on at least its surface portion. The substrate 202may comprise a single crystalline semiconductor material such as, butnot limited to Si, Ge, SiGe, GaAs, InSb, GaP, GaSb, InAlAs, InGaAs,GaSbP, GaAsSb and InP. Alternatively, the substrate 202 may include acompound semiconductor and/or an alloy semiconductor. In the illustratedembodiment, the substrate 202 is made of crystalline Si.

At operation 104, the method 100 (FIG. 1A) forms one or more epitaxiallayers over the substrate 202, as shown in FIG. 2 . In some embodiments,an epitaxial stack 212 is formed over the substrate 202. The epitaxialstack 212 includes epitaxial layers 214 of a first compositioninterposed by epitaxial layers 216 of a second composition. The firstand second composition may be different. In an embodiment, the epitaxiallayers 214 are SiGe and the epitaxial layers 216 are silicon. However,other embodiments are possible including those that provide for a firstcomposition and a second composition having different oxidation ratesand/or etch selectivity. In some embodiments, the epitaxial layer 214includes SiGe and where the epitaxial layer 216 includes silicon, thesilicon oxidation rate is less than the SiGe oxidation rate. It is notedthat five (5) layers of epitaxial layers 214 and four (4) layers ofepitaxial layers 216 are illustrated in FIG. 2 , which is forillustrative purposes only and not intended to be limiting beyond whatis specifically recited in the claims. It can be appreciated that anynumber of epitaxial layers may be formed in the epitaxial stack 212; thenumber of layers depending on the desired number of channels regions forthe device 200. In some embodiments, the number of epitaxial layers 216is between 2 and 10, such as 4 or 5.

In some embodiments, the epitaxial layer 214 has a thickness rangingfrom about 5 nm to about 15 nm. The epitaxial layers 214 may besubstantially uniform in thickness. In some embodiments, the epitaxiallayer 216 has a thickness ranging from about 5 nm to about 15 nm. Insome embodiments, the epitaxial layers 216 of the stack aresubstantially uniform in thickness. As described in more detail below,the epitaxial layer 216 may serve as channel region(s) for asubsequently-formed multi-gate device and its thickness chosen based ondevice performance considerations. The epitaxial layer 214 may serve todefine a gap between adjacent channel region(s) for asubsequently-formed multi-gate device and its thickness chosen based ondevice performance considerations.

By way of example, epitaxial growth of the epitaxial stack 212 may beperformed by a molecular beam epitaxy (MBE) process, a metalorganicchemical vapor deposition (MOCVD) process, and/or other suitableepitaxial growth processes. In some embodiments, the epitaxially grownlayers such as, the epitaxial layers 216 include the same material asthe substrate 202. In some embodiments, the epitaxially grown layers 214and 216 include a different material than the substrate 202. As statedabove, in at least some examples, the epitaxial layer 214 includes anepitaxially grown Si_(1-x)Ge_(x) layer (e.g., x is about 10˜55%) and theepitaxial layer 216 includes an epitaxially grown silicon (Si) layer.Alternatively, in some embodiments, either of the epitaxial layers 214and 216 may include other materials such as germanium, a compoundsemiconductor such as silicon carbide, gallium arsenide, galliumphosphide, indium phosphide, indium arsenide, and/or indium antimonide,an alloy semiconductor such as SiGe, GaAsP, AlInAs, AlGaAs, InGaAs,GaInP, and/or GaInAsP, or combinations thereof. As discussed, thematerials of the epitaxial layers 214 and 216 may be chosen based onproviding differing oxidation, etch selectivity properties. In variousembodiments, the epitaxial layers 214 and 216 are substantiallydopant-free (i.e., having an extrinsic dopant concentration from about 0cm⁻³ to about 1×10¹⁷ cm⁻³), where for example, no intentional doping isperformed during the epitaxial growth process. In the illustratedembodiment, the bottom layer and the top layer of the epitaxial stack212 are SiGe layers. In alternative embodiments, the bottom layer of theepitaxial stack 212 is a Si layer and the top layer of the epitaxialstack 212 is a SiGe layer (not shown).

At operation 106, the method 100 (FIG. 1A) patterns the epitaxial stack212 to form semiconductor fins 220 (also referred to as fins 220), asshown in FIG. 6 . In various embodiments, each of the fins 220 includesan upper portion of the interleaved epitaxial layers 214 and 216, and abottom portion protruding from the substrate 202. In some embodiments,the operation 106 includes forming a mask layer 218 over the epitaxialstack 212. The mask layer 218 includes a first mask layer 218 ₁ and asecond mask layer 218 ₂. The first mask layer 218 ₁ is a pad oxide layermade of a silicon oxide, which may be formed by a thermal oxidation. Thesecond mask layer 218 ₂ is made of a silicon nitride (SiN), which isformed by chemical vapor deposition (CVD), including low pressure CVD(LPCVD) and plasma enhanced CVD (PECVD), physical vapor deposition(PVD), atomic layer deposition (ALD), or other suitable process. Themask layer 218 is patterned into a mask pattern by using patterningoperations including photo-lithography and etching.

Operation 106 subsequently patterns the epitaxial stack 212 in anetching process, such as a dry etch (e.g., reactive ion etching), a wetetch, and/or other suitable process, through openings defined in thepatterned mask layer 218. The stacked epitaxial layers 214 and 216 arethereby patterned into the fins 220 with trenches between adjacent fins220. The fins 220 may include a fin 220A and a fin 220B having differentwidths. In some embodiments, the top width W1 of the fin 220A is in arange from about 6 nm to about 10 nm, while the top width W2 of the fin220B is in a range from about 16 nm to about 20 nm. Each of the fin 220Aand the fin 220B protrudes upwardly in the z-direction from thesubstrate 202 and extends lengthwise in the y-direction. Sidewalls ofthe fins 220A and 220B may be straight or inclined (not shown). In FIG.3 , two fins 220 (including one fin 220A and one fin 220B) are spacedapart along the x-direction. But the number of the fins is not limitedto two, and may be as small as one or more than two.

At operation 108, the method 100 (FIG. 1A) fills the trenches betweenadjacent fins 220 with a dielectric layer 222 and an insulating layer223 to cover top surfaces and sidewalls of the fins 220, as shown inFIG. 4 . The dielectric layer 222 may include one or more dielectricmaterials. Suitable dielectric materials for the dielectric layer 222may include silicon oxides, silicon nitrides, silicon carbides,fluorosilicate glass (FSG), low-K dielectric materials, and/or othersuitable dielectric materials. The dielectric material may be depositedby any suitable technique including thermal growth, flowable CVD (FCVD),HDP-CVD, PVD, ALD, and/or spin-on techniques. The insulating layer 223and the dielectric layer 222 have different materials. Suitableinsulating materials for the insulating layer 223 may include siliconoxides, silicon nitrides, silicon carbides, fluorosilicate glass (FSG),low-k dielectric materials, and/or other suitable dielectric materials.The insulating layer 223 material may be deposited by any suitabletechnique including thermal growth, FCVD, HDP-CVD, PVD, ALD, and/orspin-on techniques. In the illustrated embodiment, the dielectric layer222 includes silicon oxide, and an insulating layer 223 includes siliconnitride.

Operation 110 subsequently planarizes the insulating layer 223 and thedielectric layer 222 by using, for example, CMP, until top surfaces ofthe mask layer 218 are revealed, and recesses the dielectric layer 222to form shallow trench isolation (STI) features (also denoted as STIfeatures 222) and dielectric fins 223, as shown in FIG. 5 In theillustrated embodiment, the STI features 222 are formed on the substrate202, while the dielectric fins 223 are formed within the STI features222, and bottom portions of the dielectric fins 223 are surrounded bythe STI features 222. A top surface of the hybrid fin 223 may be higherthan a top surface of the STI feature 222 as illustrated in FIG. 5 .Each of the hybrid fin 223 protrudes upwardly in the z-direction fromthe STI feature 222 and extends lengthwise in the y-direction. Thedielectric fins 223 and the fins 220 are spaced apart and alternatelydisposed along the x-direction. Any suitable etching technique may beused to recess the isolation features 222 including dry etching, wetetching, RIE, and/or other etching methods, and in an exemplaryembodiment, an anisotropic dry etching is used to selectively remove thedielectric material of the isolation features 222 without etching thefins 220 and the hybrid fin 223. The mask layer 218 (shown in FIG. 4 )may also be removed before, during, and/or after the recessing of theisolation features 222. In some embodiments, the mask layer 218 isremoved by the CMP process performed prior to the recessing of theisolation features 222. In some embodiments, the mask layer 218 isremoved by an etchant used to recess the isolation features 222.

At operation 112, the method 100 (FIG. 1A) forms a sacrificial (dummy)gate structure 224, as shown in FIGS. 6A, 6B and 6C, where FIG. 6B is across-sectional view along I-I′ line of the device 200 in FIG. 6A, andFIG. 6C is a cross-sectional view along II-II′ line of the device 200 inFIG. 6A. The sacrificial gate structures 224 are formed over portions ofthe fin 220A and the fin 220B which are to be channel regions. Thesacrificial gate structures 224 define the channel regions of the GAAdevices. Each of the sacrificial gate structures 224 includes asacrificial gate dielectric layer 226 and a sacrificial gate electrodelayer 228 over the sacrificial gate dielectric layer 226. Thesacrificial gate structures 224 are formed by first blanket depositingthe sacrificial gate dielectric layer 226 over the fins 220. Asacrificial gate electrode layer 228 is then blanket deposited on thesacrificial gate dielectric layer 226 and over the fins 220. Thesacrificial gate electrode layer 228 includes silicon such aspolycrystalline silicon or amorphous silicon. The thickness of thesacrificial gate dielectric layer 226 is in a range from about 1 nm toabout 5 nm in some embodiments. The thickness of the sacrificial gateelectrode layer 228 is in a range from about 100 nm to about 200 nm insome embodiments. In some embodiments, the sacrificial gate electrodelayer is subjected to a planarization operation. The sacrificial gatedielectric layer 226 and the sacrificial gate electrode layer 228 aredeposited using CVD, including LPCVD and PECVD, PVD, ALD, or othersuitable process. Subsequently, a mask layer 230 is formed over thesacrificial gate electrode layer. The mask layer 230 may include asilicon oxynitride layer 230 ₁ and a silicon nitride mask layer 230 ₂having a thickness, for example, ranging from about 10 nm to about 30 nmand ranging from about 50 nm to about 70 nm, respectively. Subsequently,a patterning operation is performed on the mask layer 230 andsacrificial gate dielectric and electrode layers are patterned into thesacrificial gate structures 224. By patterning the sacrificial gatestructures 224, the fins 220 are partially exposed on opposite sides ofthe sacrificial gate structures 224, thereby defining source/drain (S/D)regions. In this disclosure, a source and a drain are interchangeablyused and the structures thereof are substantially the same.

At operation 114, the method 100 (FIG. 1A) forms spacers 232 onsidewalls of the sacrificial gate structure 224 and sidewalls of the fin220A and the fin 220B, as shown in FIGS. 6A, 6B and 6C. The spacers 232may include a dielectric material such as silicon oxide, siliconnitride, silicon carbide, silicon oxynitride, SiCN films, siliconoxycarbide, SiOCN films, and/or combinations thereof. In someembodiments, the spacers 232 include multiple layers, such as linerlayers 232 ₁ and main spacer walls 232 ₂. By way of example, the gatesidewall spacers 232 may be formed by depositing a liner material layerand a dielectric material layer over the sacrificial gate structure 224using processes such as, a subatmospheric CVD (SACVD) process, aflowable CVD process, an ALD process, a PVD process, or other suitableprocess respectively, as shown in FIG. 6A. In some embodiments, thedeposition of the liner material layer and the dielectric material layerare followed by an etching-back (e.g., anisotropically) process toexpose portions of the fin 220A and the fin 220B adjacent to and notcovered by the sacrificial gate structure 224 (e.g., S/D regions). Theliner material layer and the dielectric material layer may remain on thesidewalls of the sacrificial gate structure 224 as gate sidewallspacers, and on the sidewalls of the fin 220A and the fin 220B as finsidewall spacers. In some embodiments, the etching-back process mayinclude a wet etch process, a dry etch process, a multiple-step etchprocess, and/or a combination thereof. The spacers 232 may have athickness ranging from about 5 nm to about 20 nm, and the liner layers232 ₁ and the main spacer walls 232 ₂ may have a thickness ranging, forexample, from about 2.5 nm to about 10 nm, respectively.

At operation 116, the method 100 (FIG. 1A) recesses a portion of the fin220A to form source/drain spaces 233A with recesses 234A in the S/Dregions, and a second portion of the fin 220B to form the othersource/drain spaces 233B with recesses 234B in the other S/D regions asshown in FIGS. 7A, 7B and 7C, where FIG. 7B is a cross-sectional viewalong I-I′ line of the device 200 in FIG. 7A, and FIG. 7C is across-sectional view along II-II′ line of the device 200 in FIG. 7A. Thestacked epitaxial layers 214 and 216 are etched down at the S/D regions.In many embodiments, the operation 114 forms the source/drain spaces233A and the source/drain spaces 233B by a suitable etching process,such as a dry etching process, a wet etching process, or an RIE process.The etching process at operation 114 may implement a dry etching processusing an etchant including a bromine-containing gas (e.g., HBr and/orCHBR₃), a fluorine-containing gas (e.g., CF₄, SF₆, CH₂F₂, CHF₃, and/orC₂F₆), oxygen-containing gas (e.g., O₂), a chlorine-containing gas(e.g., Cl₂), a helium-containing gas (e.g., He), an argon-containing gas(e.g., Ar), other suitable gases, or combinations thereof. During theetching process at the operation 114, the spacers 232 at sidewalls ofthe portions of the fins 220A and 220B are also recessed, therebyforming spacers 232A and 232B. In some embodiments, the spacer 232B hasa height greater than a height of the spacer 232A. The spacers 232A anda surface of the fin 220A form a recess 234A, and the spacers 232B andtop surfaces of the fin 220B form a recess 234B. Since the width W1 ofthe fin 220A is less than the width W2 of the fin 220B, a width W1′ ofthe recess 234A is less than a width W2′ of the recess 234B. In someembodiments, the width W1′ is in a range from about 6 nm to about 10 nm,while the width W2′ is in a range from about 16 nm to about 20 nm.

At operation 118, the method 100 (FIG. 1B) forms inner spacers directlyunder the gate sidewall spacers 232. In some embodiments, the operation116 first laterally etches the epitaxial layers 214 in the y-direction,thereby forming cavities 236, as shown in FIGS. 8A, 8B and 8C, whereFIG. 8B is a cross-sectional view along A-A line of the device 200 inFIG. 8A, and FIG. 8C is a cross-sectional view along II-II′ line of thedevice 200 in FIG. 8A. The amount of etching of the epitaxial layers 214is in a range from about 1 nm to about 4 nm in some embodiments. Theepitaxial layers 214 may be selectively etched by using a wet etchantsuch as, but not limited to, ammonium hydroxide (NH₄OH),tetramethylammonium hydroxide (TMAH), ethylenediamine pyrocatechol(EDP), potassium hydroxide (KOH), HF, O₃, H₂O₂, or HCl solutions.Alternatively, the operation 116 may first selectively oxidize lateralends of the epitaxial layers 214 that are exposed in the recesses 234 toincrease the etch selectivity between the epitaxial layers 214 and 216.In some examples, the oxidation process may be performed by exposing thedevice 200 to a wet oxidation process, a dry oxidation process, or acombination thereof.

Subsequently, operation 116 forms an inner spacer material layer 238 onthe lateral ends of the epitaxial layer 214, on the epitaxial layers216, on the dielectric fins 223, and in cavities 236 and the recesses234A and 234B, as shown in FIGS. 9A, 9B and 9C, where FIG. 9B is across-sectional view along I-I′ line of the device 200 in FIG. 9A, andFIG. 9C is a cross-sectional view along II-II′ line of the device 200 inFIG. 9A. The inner spacer material layer 238 may include silicon oxides,silicon nitrides, silicon carbides, silicon carbide nitride, siliconoxide carbide, silicon carbide oxynitride, and/or other suitabledielectric materials. In some embodiments, the inner spacer materiallayer 238 is deposited as a conformal layer. The inner spacer materiallayer 238 may be formed by ALD or any other suitable method. Byconformally forming the inner spacer material layer 238, the size of thecavity 234 is reduced or the cavity 234 is completely filled. The innerspacer material layer 238 may have a thickness ranging from about 4 nmto about 6 nm, for example. In some embodiments, portions of the innerspacer material layer 238 on sidewalls of the recess 234A are merged sothat the recess 234A is completely filled with the inner spacer materiallayer 238, while portions of the inner spacer material layer 238 onsidewalls of the recess 234B are not merged so that the recess 234B isnot completely filled with the inner spacer material layer 238.

After the inner spacer material layer 238 is formed, an etching processis performed to partially remove the inner spacer material layer 238, asshown in FIGS. 10A, 10B and 10C, where FIG. 10B is a cross-sectionalview along I-I′ line of the device 200 in FIG. 10A, and FIG. 10C is across-sectional view along II-II′ line of the device 200 in FIG. 10A. Bythis etching, the inner spacer material layer 238 remains substantiallywithin the cavity 236, because of a small volume of the cavity.Similarly, the inner spacer material layer 238 remains substantiallywithin the recess 234A, because of a small volume of the recess 234A.Generally, plasma dray etching etches a layer in wide and flat areasfaster than a layer in concave (e.g., holes, grooves, recesses and/orslits) portions. Thus, the inner spacer material layer 238 may remaininside the cavities 236 and also remain in the recesses 234A, while theinner spacer material layer 238 may be removed from the recesses 234B.The remained portions of the inner spacer material layer 238 in thecavities 236 are denoted as the inner spacers 238. The remained portionsof the inner spacer material layer 238 in the recess 234A are denoted asresidues 238R (or referred to as inner spacer residues).

At operation 120, the method 100 (FIG. 1B) performs a treatment processto remove the residue 238R in the recess 234A, so that the fin 220A isexposed, as shown in FIGS. 10A, 10B and 10C. In some embodiments, thetreatment process may be performed by a selectively etching process. Theselectively remove process may be performed by using photo-lithographyand etching. As an example to perform the selectively etching process, aphotoresist 239 is formed and patterned on the substrate 202, as shownin FIGS. 10A and 10C. The photoresist 239 may be formed by spin coatingor the like and may be exposed to light for patterning. The patterningforms openings O1 through the photoresist 239 to expose the residue 238Rand the spacer 232A. After the photoresist 239 is patterned, thephotoresist 239 may be referred to as a patterned mask layer. Theresidue 238R and the spacer 232A are removed by using an acceptableetching process, such as by an anisotropic etching process. In someembodiments, the anisotropic etching process may implement a dry etchingprocess using an etchant including a bromine-containing gas (e.g., HBrand/or CHBR₃), a fluorine-containing gas (e.g., CF₄, SF₆, CH₂F₂, CHF₃,and/or C₂F₆), oxygen-containing gas (e.g., O₂), a chlorine-containinggas (e.g., Cl₂), a helium-containing gas (e.g., He), an argon-containinggas (e.g., Ar), other suitable gases, or combinations thereof.Thereafter, the photoresist 239 is removed. The photoresist 113 may beremoved by an acceptable ashing or stripping process, such as using anoxygen plasma or the like, for example.

In some embodiments, the entire spacers 232A and the residues 238R areremoved, sidewalls of the dielectric fins 223, a surface of the STIfeatures 222A that are adjacent to the fin 220A, and the surface of thefin 220A are exposed by the source/drain spaces 233A, as shown in FIGS.11A and 11C, where FIG. 11B is a cross-sectional view along I-I′ line ofthe device 200 in FIG. 11A, and FIG. 11C is a cross-sectional view alongII-II′ line of the device 200 in FIG. 11A. A bottom of the recess 234Bof the source/drain spaces 233B exposes a surface of the fin 220B. Insome embodiments, a lower width W1″ of the source/drain spaces 233A maybe larger than a lower width W2′ of the recess 234B. Height of bottomsurface of the source/drain spaces 233A may be lower than height ofbottom surface of the recess 234B because the fin 220A and the STIfeatures 222A may have some loss during the selectively etching processat the operation 120 or a cleaning process (such as a SiCoNi process)for removing an native oxide after the selectively etching process. Insome embodiments, the surface of the STI features 222A is about 2 nm toabout 10 nm lower than the surface of the STI features 222B.

In alternative embodiments, the height of the spacers 232A is reduced sothat some spacers 232A′ may remain on the STI features 222A, while theentire residues 238R are removed to expose the fin 220A during theelectively etching process at the operation 120, as shown in FIG. 11D.Sidewalls of the spacers 232A′ and the surface of the fin 220A form arecess 234A′. The spacers 232A′ and the spacers 232B have differentshapes and layers. In some embodiments, the spacer 232A′ includes theliner material layers 232 ₁, while the spacer 232B includes the linermaterial layer 232 ₁ the dielectric material layer 232 ₂ embedded in theliner material layer 232 ₁. The cross-sectional shape of the linermaterial layers 232 ₁ of the spacer 232A′ has an “-” shape, for example,while the cross-sectional shape of the liner material layer 232 ₁ of thespacer 232 has an “U” shape, and the cross-sectional shape of thedielectric material layer 232 ₂ of the spacer 232 has an “I” shape. Theheight of the spacers 232A′ is less than a height of the spacers 232B.In some embodiments, the spacer 232A′ has the height in a range fromabout 5 nm to about 10 nm, and the spacer 232B has the height in a rangefrom about 10 nm to about 30 nm. The width W1′ of the recess 234A′ maybe less than the width W2′ of the recess 234B. In some embodiments, thewidth W1′ is in a range from about 6 nm to about 10 nm, while the widthW2′ is in a range from about 16 nm to about 20 nm. The aspect ratio ofthe recess 234A′ may be less than 1.7. In some embodiments, the aspectratio of the recess 234A′ is in a range from 0 to 1.7.

At operation 122, the method 100 (FIG. 1B) forms epitaxial S/D features240A and 240B in the source/drain spaces 233A and 233B, respectively. Insome embodiments, the epitaxial S/D features 240A and 240B includesilicon for an n-type FET and SiGe for a p-type FET. The epitaxial S/Dfeatures 240A and 240B may be formed by an epitaxial growth method usingCVD, ALD or molecular beam epitaxy (MBE). Since the residue 238R isremoved, the epitaxial S/D features 240A and 240B can grow from the fins220A and 220B at the bottom of the source/drain spaces 233A, and fromthe epitaxial layers 216 at the sidewalls of the source/drain spaces233A. Therefore, the epitaxial S/D features 240A and 240B have fewermerge defect and good quality.

The epitaxial S/D features 240A and 240B are formed in contact with theepitaxial layers 216 of the fins 220, and separated from the epitaxiallayers 214 of the fins 220 by the inner spacers 238 respectively, andthe epitaxial S/D features 240A are separated from the epitaxial S/Dfeatures 240B by the dielectric fins 223, as shown in FIGS. 12A, 12B and12C, where FIG. 12B is a cross-sectional view along I-I′ line of thedevice 200 in FIG. 12A, and FIG. 12C is a cross-sectional view alongII-II′ line of the device 200 in FIG. 12A. In some embodiment, cavities236 are not filled by the inner spacer material 238 but be capped by theepitaxial S/D features 240A or 240B. Therefore, cavities 236 is alsoreferred to as “air spacer”.

In the illustrated embodiments, the epitaxial S/D feature 240A and theepitaxial S/D feature 240B have different shapes, as shown in FIGS. 12C,12D and 12E. The cross-sectional shape of the epitaxial S/D feature 240Ais octagonal, for example. the epitaxial S/D feature 240A is inphysically contact with the fin 220A and the hybrid fin 223. In someembodiments, the epitaxial S/D feature 240A has a bottom surface BS-1,lower facets LF-1, sidewalls S-1, upper facets UF-1 and a top surfaceTS-1. The bottom surface BS-1 is in physically contact with the surfaceof the fin 220A. A first edge of the lower facet LF-1 of the epitaxialS/D feature 240A is connected to an edge of the bottom surface BS-1 ofthe epitaxial S/D feature 240A and in physically contact with the fin220A and the STI feature 222A. A second edge of the lower facets LF-1 ofthe epitaxial S/D feature 240A is connected to a bottom edge of thesidewall S-1 of the epitaxial S/D feature 240A and in physically contactwith the hybrid fin 223. The sidewalls S-1 of the epitaxial S/D feature240A are in physically contact with portion sidewalls of the dielectricfins 223. A first edge of the upper facet UF-1 of the epitaxial S/Dfeature 240A is connected to a top edge of the sidewall S-1 of theepitaxial S/D feature 240A and in physically contact with the hybrid fin223. A second edge of the upper facets UF-1 of the epitaxial S/D feature240A is connected to an edge of a top surface TS-1 of the epitaxial S/Dfeature 240A. In some embodiments, the top surface TS-1 and the bottomsurface B-1 of the epitaxial S/D feature 240A are {100} facets, thelower facets LF-1 and the upper facets UF-1 of the epitaxial S/D feature240A are {111} facets, and the sidewalls S-1 of the epitaxial S/Dfeature 240A are {110} facets. In addition, a surface of the lower facetLF-1, the top surface of the STI feature 222A and the other portionsidewall of the hybrid fin 223 form an air gap AG1, and a shape of theair gap AG1 may be, for example, a triangle. The sidewalls S-1 of theepitaxial S/D feature 240A may not be vertical because the dielectricfins 223 may be damaged during the aforementioned etching process. Insome embodiments, top sidewalls of the hybrid fin 223 are damaged, sothat an upper width of the hybrid fin 223 is less than a lower widththereof. Therefore, upper portion of the sidewalls S-1 may be greaterthan lower portion thereof.

The epitaxial S/D feature 240B has a base portion BPB on the fin 220B,while the epitaxial S/D feature 240A does not have base portion, asshown in FIG. 12C. The epitaxial S/D feature 240B further has a mainportion MPB on the base portion BPB. The base portion BPB has arectangular cross-sectional shape, and the main portion MPB has ahexagonal cross-sectional shape, for example. The base portion BPB has abottom surface BS-2 and sidewalls S-2. The bottom surface BS-2 of thebase portion BPB is in physically contact with the surface of the fin220B. The sidewalls S-2 of the base portion BP are in physically contactwith the spacers 232B. A bottom edge of the sidewall S-2 of the baseportion BP is connected to an edge of the bottom surface BS-2 of thebase portion BPB and in physically contact with the second fin 220B andthe STI feature 222B. A top edge of the sidewall S-2 of the base portionBPB is connected to the main portion MPB and in physically contact withthe spacer 232B. A bottom of the main portion MPB is connected to a topof the base portion BPB. The main portion MPB has lower facets LF-2,upper facets UF-2, and a top surface TS-2. The lower facet LF-2 isconnected to a top edge of the sidewall S-2 of the base portion BP, andthe upper facet UF-2 is connected to an edge of the top surface TS-2.The lower facet LF-2 and the upper facet UF-2 are in physically contactwith each other. In some embodiments, the bottom surface BS-2 of thebase portion BP and the top surface TS-2 of the of the main portion MPBare {100} facets, the sidewall S-2 of the base portion BP are {110}facets, and the lower facets LF-2 and the upper facets UF-2 of the mainportion MPB are {111} facets.

In some embodiments, the lower facets LF-2 and the lower facets LF-2 ofthe main portion MPB are in physically contact with the dielectric fins223, as shown in FIGS. 12C and 12D. In addition, a surface of the lowerfacets LF-2 of the main portion MP, a top surface of the spacers 232B,and a portion sidewall of the hybrid fin 223 form an air gap AG3, and ashape of the air gap AG3 may be, for example a triangle. In alternativeembodiments, the lower facets LF-2 and the lower facets LF-2 of the mainportion MPB does not contact the hybrid fin 223, as shown in FIG. 12E.

In alternative embodiments in which the spacers 232A′ remain on the STIfeatures 222A as shown in FIG. 11D, the epitaxial S/D feature 240Aincludes a base portion BPA formed in the recess 234A′ and a mainportion MPA on the base portion BPA, as shown in FIGS. 12D and 12E. Abottom surface BS-1 of the base portion BPA is in physically contactwith the fin 220A. Sidewalls S-1′ of the base portion BPA are inphysically contact with the spacers 232A′. First edges of lower facetsLF-1 of the main portion MPA are in physically contact with the baseportion BPA, and second edges of the lower facets LF-1 of the mainportion MPA are in physically contact with the hybrid fin 223. Inaddition, a surface of the lower facets LF-1 of the main portion MPA, atop surface of the spacers 232A′, and a portion sidewall of the hybridfin 223 form an air gap AG2, and a shape of the air gap AG2 may be, forexample, a triangle.

The bottom surface BS-1 of the epitaxial S/D feature 240A may be equalto or lower than the bottom surface BS-2 of the epitaxial S/D feature240B, as shown in FIGS. 12C to 12E. The height H1 of the epitaxial S/Dfeature 240A may be equal to or greater than the height H2 of theepitaxial S/D feature 240B, as shown in FIGS. 12C to 12E. The contactareas of the epitaxial S/D feature 240A and the dielectric fins 223 maybe greater than the contact areas of the epitaxial S/D feature 240B andthe spacers 232B, as shown in FIG. 12C. The contact areas of theepitaxial S/D feature 240A and the spacers 232A′ are less than contactareas of the epitaxial S/D feature 240B and the spacers 232B, as shownin FIG. 12D or 12E. The contact areas of the epitaxial S/D feature 240Aand the dielectric fins 223 is greater than the contact areas of theepitaxial S/D feature 240B and the dielectric fins 223, as shown inFIGS. 12C and 12D.

At operation 124, the method 100 (FIG. 1B) forms a contact etch stoplayer (CESL) 242 over the epitaxial S/D features 240A and 240B and aninterlayer dielectric (ILD) layer 244 over the CESL layer 242, as shownin FIGS. 13A, 13B and 13C, where FIG. 13B is a cross-sectional viewalong I-I′ line of the device 200 in FIG. 13A, and FIG. 13C is across-sectional view along II-II′ line of the device 200 in FIG. 13A.The CESL layer 242 may comprise silicon nitride, silicon oxynitride,silicon nitride with oxygen (O) or carbon (C) elements, and/or othermaterials; and may be formed by CVD, PVD (physical vapor deposition),ALD, or other suitable methods. The ILD layer 244 may comprisetetraethylorthosilicate (TEOS) oxide, un-doped silicate glass, or dopedsilicon oxide such as borophosphosilicate glass (BPSG), fused silicaglass (FSG), phosphosilicate glass (PSG), boron doped silicon glass(BSG), and/or other suitable dielectric materials. The ILD layer 244 maybe formed by PECVD or FCVD (flowable CVD), or other suitable methods. Insome embodiments, forming the ILD layer 244 further includes performinga CMP process to planarize a top surface of the device 200, such thatthe top surfaces of the sacrificial gate structure 224 are exposed.

At operation 126 the method 100 (FIG. 1B) removes the sacrificial gatestructures 224 to form gate trenches 254, as shown in FIGS. 13A, 13B and13C. The ILD layer 244 and the CESL layer 242 protects the epitaxial S/Dfeatures 240A and 240B during the removal of the sacrificial gatestructure 224. The sacrificial gate structure 224 may be removed usingplasma dry etching and/or wet etching. When the sacrificial gateelectrode layer 228 is polysilicon and the ILD layer 244 is siliconoxide, a wet etchant such as a TMAH solution may be used to selectivelyremove the sacrificial gate electrode layer. The sacrificial gatedielectric layer 226 is thereafter removed using plasma dry etchingand/or wet etching.

At operation 128, the method 100 (FIG. 1B) releases channel members fromthe channel region of the GAA device 200, as shown in FIGS. 14A, 14B and14C, where FIG. 14B is a cross-sectional view along I-I′ line of thedevice 200 in FIG. 14A, and FIG. 14C is a cross-sectional view alongII-II′ line of the device 200 in FIG. 14A. In the illustratedembodiment, channel members are epitaxial layers 216 in the form ofnanosheets. In the present embodiment, the epitaxial layers 216 includesilicon, and the epitaxial layers 214 include silicon germanium. Theplurality of epitaxial layers 214 may be selectively removed. In someimplementations, the selectively removal process includes oxidizing theplurality of epitaxial layers 214 using a suitable oxidizer, such asozone. Thereafter, the oxidized epitaxial layers 214 may be selectivelyremoved. To further this embodiment, the operation 128 includes a dryetching process to selectively remove the epitaxial layers 214, forexample, by applying an HCl gas at a temperature of about 0° C. to about150° C., or applying a gas mixture of CF₄, SF₆, CHF₃, HF, NH₃, F₂, ClF₃,N₂, Ar, and/or He. For the sake of simplicity and clarity, afteroperation 130, the epitaxial layers 216 are denoted as nanosheets 216.At this point, as shown in FIG. 14A, vertically stacked nanosheets 216are formed in the channel regions of GAA device.

At operation 130, the method 100 (FIG. 1B) forms metal gate structure258 in the gate trench 254 engaging the nanosheets 216 in the channelregion, as shown in FIGS. 15A, 15B and 15C, where FIG. 15B is across-sectional view along I-I′ line of the device 200 in FIG. 15A, andFIG. 15C is a cross-sectional view along II-II′ line of the device 200in FIG. 15A. A gate dielectric layer 262 is formed wrapping eachnanosheet 216 in the channel region, and a gate electrode layer 264 isformed on the gate dielectric layer 262. The inner spacers 238 separatethe metal gate structure 258 from the epitaxial S/D features 240.

In some embodiments, the gate dielectric layer 262 includes one or morelayers of a dielectric material, such as silicon oxide, silicon nitride,or high-k dielectric material, other suitable dielectric material,and/or combinations thereof. Examples of high-k dielectric materialinclude HfO₂, HfSiO, HfSiON, HfTaO, HfSiO, HfZrO, zirconium oxide,aluminum oxide, titanium oxide, hafnium dioxide-alumina (HfO₂—Al₂O₃)alloy, other suitable high-k dielectric materials, and/or combinationsthereof. In some embodiments, the gate dielectric layer 262 includes aninterfacial layer (not shown) formed between the channel layers and thedielectric material. The gate dielectric layer 262 may be formed by CVD,ALD or any suitable method. In one embodiment, the gate dielectric layer262 is formed using a highly conformal deposition process such as ALD inorder to ensure the formation of a gate dielectric layer having auniform thickness around each channel layers. The thickness of the gatedielectric layer 262 is in a range from about 1 nm to about 6 nm in someembodiments.

The gate electrode layer 264 is formed on the gate dielectric layer 262to surround each channel layer. The gate electrode layer 264 includesone or more layers of conductive material, such as polysilicon,aluminum, copper, titanium, tantalum, tungsten, cobalt, molybdenum,tantalum nitride, nickel silicide, cobalt silicide, TiN, WN, TiAl,TiAlN, TaCN, TaC, TaSiN, metal alloys, other suitable materials, and/orcombinations thereof. The gate electrode layer 264 may be formed by CVD,ALD, electro-plating, or other suitable method. In certain embodimentsof the present disclosure, one or more work function adjustment layers(not shown) are interposed between the gate dielectric layer 262 and thegate electrode 264. The work function adjustment layers are made of aconductive material such as a single layer of TiN, TaN, TaAlC, TiC, TaC,Co, Al, TiAl, HfTi, TiSi, TaSi or TiAlC, or a multilayer of two or moreof these materials. For the n-channel FET, one or more of TaN, TaAlC,TiN, TiC, Co, TiAl, HfTi, TiSi and TaSi is used as the work functionadjustment layer, and for the p-channel FET, one or more of TiAlC, Al,TiAl, TaN, TaAlC, TiN, TiC and Co is used as the work functionadjustment layer. The work function adjustment layer may be formed byALD, PVD, CVD, e-beam evaporation, or other suitable process. Further,the work function adjustment layer may be formed separately for then-channel FET and the p-channel FET which may use different metallayers.

The gate dielectric layer 262, the work function adjustment layer, andthe gate electrode layer 264 may also be deposited over the uppersurfaces of the ILD layer 244 and the CESL layer 242. The gatedielectric layer 262 and the gate electrode layer 264 formed over theILD layer 244 and the CESL layer 242 are then planarized by using, forexample, CMP, until the top surfaces of the ILD layer 244 and the CESLlayer 242 is revealed. In some embodiments, after the planarizationoperation, the gate electrode layer 264 is recessed and a cap insulatinglayer (not shown) is formed over the recessed gate electrode 264. Thecap insulating layer includes one or more layers of a siliconnitride-based material, such as SiN. The cap insulating layer may beformed by depositing an insulating material followed by a planarizationoperation.

Although not intended to be limiting, one or more embodiments of thepresent disclosure provide many benefits to a semiconductor device andthe formation thereof. The present disclosure provides methods offorming a GAA device including a selectively etching process to removeinner spacer reside. Accordingly, this provides a benefit of epitaxialS/D feature with fewer merge defect, better strain, and higher mobility,and better device performance. Furthermore, the GAA flow withselectively etching process may be easily integrated into existingsemiconductor fabrication processes.

In some embodiment, the present disclosure is directed to a method ofmanufacturing a semiconductor device. The method comprises providing asemiconductor substrate; forming an epitaxial stack on the semiconductorsubstrate, wherein the epitaxial stack comprises a plurality of firstepitaxial layers interposed by a plurality of second epitaxial layers;patterning the epitaxial stack and the semiconductor substrate to form asemiconductor fin; recessing a portion of the semiconductor fin to formsource/drain spaces; laterally removing portions of the plurality offirst epitaxial layers exposed by the source/drain spaces to form aplurality of cavities; forming inner spacers in the plurality ofcavities; performing a treatment process to remove an inner spacerresidue in the source/drain spaces; forming S/D features in thesource/drain spaces; and forming a gate structure engaging thesemiconductor fin.

In another some embodiments, the present disclosure is directed to amethod of manufacturing a multi-gate device. The method comprisesproviding a semiconductor substrate; forming a plurality ofsemiconductor nanosheets on a first portion of the semiconductorsubstrate; forming an epitaxial source/drain (S/D) feature on a secondportion of the semiconductor substrate, wherein the epitaxialsource/drain (S/D) feature is in contact with the plurality ofsemiconductor nanosheets; forming a gate structure wrapping each of theplurality of semiconductor nanosheets; and before the forming theepitaxial source/drain (S/D) feature, performing a treatment process toremove a residue on a surface of the second portion of the semiconductorsubstrate.

In yet another embodiments, the present disclosure is directed to asemiconductor device. The semiconductor device comprises a semiconductorsubstrate; a plurality of semiconductor nanosheets disposed over a firstportion of the semiconductor substrate; a plurality of source/drain(S/D) features on a second portion of the semiconductor substrate andabutting the plurality of semiconductor nanosheets; and an isolationfeature around the second portion of the semiconductor substrate,wherein there is an air gap between the isolation feature and thesource/drain (S/D) feature, and a top surface of the isolation featureand a lower facet of the source/drain (S/D) feature are exposed by theair gap.

The foregoing has outlined features of several embodiments so that thoseskilled in the art may better understand the detailed description thatfollows. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A semiconductor device, comprising: asemiconductor substrate comprising a first fin and a second fin, whereinthe first fin has a first width less than a second width of the secondfin, and a top surface of the first fin is lower than a top surface ofthe second fin; a plurality of semiconductor nanosheets disposed on thefirst fin and the second fin; a plurality of source/drain (S/D) featureson the first fin and the second fin and abutting the plurality ofsemiconductor nanosheets; and a gate stack wrapping each of theplurality of semiconductor nanosheets.
 2. The semiconductor device ofclaim 1, wherein a bottom surface of the plurality of source/drain (S/D)features on the first fin is lower than a bottom surface of theplurality of source/drain (S/D) features on the second fin.
 3. Thesemiconductor device of claim 1, wherein a height of the plurality ofsource/drain (S/D) features on the first fin is larger than a height ofthe plurality of source/drain (S/D) features on the second fin.
 4. Thesemiconductor device of claim 1, wherein the plurality of source/drain(S/D) features on the first fin is isolated from the plurality ofsource/drain (S/D) features on the second fin through a hybrid fin. 5.The semiconductor device of claim 4, wherein a first sidewall of thehybrid fin is exposed by a first air gap laterally aside the pluralityof source/drain (S/D) features on the first fin, and a second sidewallof the hybrid fin is exposed by a second air gap located laterally asidethe plurality of source/drain (S/D) features on the second fin, and thefirst sidewall is lower than the second sidewall.
 6. The semiconductordevice of claim 4, wherein the plurality of source/drain (S/D) featureson the second fin have base portions connected to the second fin, andthe first source/drain (S/D) feature is free from base portion.
 7. Thesemiconductor device of claim 6, further comprising spacers laterallybetween the base portions and the hybrid fin.
 8. The semiconductordevice of claim 4, wherein the plurality of source/drain (S/D) featureson the first fin have first base portions, the plurality of source/drain(S/D) features on the second fin have second base portions, and a heightof the first base portions are lower than a height of the second baseportions.
 9. The semiconductor device of claim 8, further comprising:first spacers surround the first base portions; and second spacerssurround the second base portions, and a height of the first spacers arelower than a height of the second spacers.
 10. A semiconductor device,comprising: a semiconductor substrate comprising a first fin and asecond fin, wherein the first fin has a first width less than a secondwidth of the second fin; an isolation feature around the first fin andthe second fin, wherein a surface of the isolation feature around thefirst fin is lower than a second surface of the isolation feature aroundthe second fin; a plurality of gate stack over the first fin and thesecond fin; and a plurality of first source/drain (S/D) features on thefirst fin and a plurality of second source/drain (S/D) features on thesecond fin.
 11. The semiconductor device of claim 10, wherein a bottomsurface of the plurality of first source/drain (S/D) features is lowerthan a bottom surface of the plurality of second source/drain (S/D)features.
 12. The semiconductor device of claim 11, further comprising ahybrid fin isolates the plurality of first source/drain (S/D) featuresfrom the plurality of second source/drain (S/D) features.
 13. Thesemiconductor device of claim 12, wherein a lower portion of the hybridfin is surrounded by the isolation feature.
 14. The semiconductor deviceof claim 10, further comprising spacers, wherein the spacers surroundbase portions of the plurality of second source/drain (S/D) features.15. The semiconductor device of claim 11, wherein the plurality of firstsource/drain (S/D) features, the hybrid fin, and the isolation featureform first air gaps, and the plurality of second source/drain (S/D)features, the hybrid fin, and the spacers form second air gaps, and thefirst air gaps are lower than the second air gaps.
 16. A method ofmanufacturing a multi-gate device, comprising: providing a semiconductorsubstrate, wherein the semiconductor substrate has a first fin and asecond fin, the first fin has a first width less than a second width ofthe second fin; forming a plurality of semiconductor nanosheets on thefirst fin and the second fin; performing a selectively etching processto remove a portion of the first fin so that a top surface of the firstfin is lower than a top surface of the second fin; forming an epitaxialsource/drain (S/D) feature on the first fin and the second fin andabutting the plurality of semiconductor nanosheets; and forming a gatestructure wrapping each of the plurality of semiconductor nanosheets.17. The method of claim 16, further comprising forming an isolationfeature around the first fin and the second fin, wherein the selectivelyetching process is performed to further expose a surface of theisolation feature around the first fin.
 18. The method of claim 16,wherein the selectively etching process is performed to further remove aportion of the isolation feature around the first fin.
 19. The method ofclaim 16, wherein the forming a plurality of semiconductor nanosheetscomprises: forming a plurality of first layers interposed by a pluralityof second layers on first portion of the semiconductor substrate;laterally removing portions of the plurality of first layers to form aplurality of cavities; forming inner spacers in the plurality ofcavities; and removing the first layers, wherein the selectively etchingprocess is performed to further remove a residue on the first finem andthe residue is formed during the forming inner spacers.
 20. The methodof claim 16, further comprising: forming a sacrificial gate structure onthe first fin and the second fin; and replace the sacrificial gatestructure by forming the gate structure.